In recent years, there has been a growing need for gigabit-class high-speed radio used in an indoor environment. For example, the use of a high-frequency band (e.g., 60 GHz) has been promoted since it facilitates broadband transmission compared to a microwave band equal to or smaller than about 6 GHz which has been conventionally used. On the other hand, a radio wave in such a high-frequency band has characteristics that it has small diffraction and strong rectilinear propagation properties. Thus, when there is an obstruction between communication apparatuses that transmit and receive radio waves in the high-frequency band, there are caused problems that communication quality is deteriorated, and in particular, communication is interrupted in a millimeter waveband.
In order to solve the problems, for example, such a measure is taken to maintain the communication quality by using a reflected wave instead of using a direct wave when there is an obstruction between the communication apparatuses as described above. Meanwhile, it may be possible that the phase of an input radio wave by the reflected wave is inverted. Thus, the use of a circularly polarized wave in a transmitting antenna and a receiving antenna may dramatically decrease the reception power.
Accordingly, a linearly polarized wave is typically used in the reflected wave communication stated above. In this case, there are two main problems as follows. The first problem is that, when linearly polarized wave antennas are used in a transmitter and a receiver, the reception sensitivity becomes maximum if the polarization directions of the antennas are uniformly oriented, whereas the reception sensitivity may be deteriorated if there are deviations in the polarization directions. Further, when the reflected wave communication is executed in an indoor area (in particular, home environment), if there is a restriction in the positional relation in which the transmitter and the receiver are installed and it is required to keep the angles of the transmitting and receiving antennas constant in order to prevent this problem, it may dramatically impair convenience.
The second problem is that a reflectance of a reflector greatly varies according to the incident angle of the wave and the polarization direction that is used. For example, when a parallel polarized wave is used, it may be possible that the reception sensitivity cannot be obtained at a specific incident angle corresponding to Brewster's angle. This is because the reflectance of the reflector depends on the angle between the electrical field excitation direction of the linearly polarized wave and the reflection surface. In general, the reflection surface in an indoor area includes not only a horizontal or vertical reflection surface such as walls or floors but also an oblique reflection surface such as a sofa arranged indoors. Furthermore, since the environment in which radio waves propagate is easy to change in an indoor area due to the exit and entry of people, for example, it is preferable to secure a plurality of communication paths. In such a case, various reflection surfaces are used for each of the communication paths. Accordingly, in order to solve the two problems, it is required to vary the polarization direction. Further, when the polarized wave of the radio wave emitted from the communication partner is unknown, it is required not only to generate a linearly polarized wave, but also to generate right-hand and left-hand circularly (or elliptically) polarized waves.
A method of changing the polarization direction includes a method of arranging a plurality of excitation units in an antenna, for example. Further, this method of arranging the plurality of excitation units includes a method of adjusting input power and an input phase difference to each excitation unit and a method of switching the excitation units for each desired polarization wave. Among them, the former method is easier in its creation method and usage, and has been widely used.
For example, as shown in FIG. 8, a related antenna apparatus 800 includes a high-frequency source 801, a branch circuit 802, phase shifters 803, power supply lines 804, and a patch antenna 805. A high-frequency signal output from the high-frequency source 801 is divided by the branch circuit 802, and then input to the patch antenna 805 via the phase shifters 803 and the power supply lines 804. Two excitation units on the patch antenna 805 connected to the respective power supply lines 804 excite radiation electric fields that are orthogonal to each other. The phases of the high-frequency signals input to the two excitation units have a phase difference of 0°, 90°, 180°, and 270°, for example, by the phase shifters 803. For example, when the linearly polarized wave is generated, the phase difference of 0 or 180 degrees is provided, and when the circularly polarized wave is generated, the phase difference of 90 or 270 degrees is provided.
Further, an antenna apparatus including an antenna element for emitting two orthogonal linearly polarized waves from two excitation units and a phase shifter for adjusting a phase difference for every 90 degrees is known (e.g., see PTL 1).